Remove paren_depth global

Remove paren_depth global

This removes the "paren_depth" global.  In most cases, it is made into
a static global in a given parser.  I consider this a slight
improvement, because it makes it clear that the variable isn't used
for communication between different modules of gdb.  The one exception
is the Rust parser, which already incorporates all local state into a
transient object; in this case the parser depth is now a member.

gdb/ChangeLog
2019-04-04  Tom Tromey  <tom@tromey.com>

	* rust-exp.y (struct rust_parser) <paren_depth>: New member.
	(rustyylex, rust_lex_test_init, rust_lex_test_one)
	(rust_lex_test_sequence, rust_lex_test_push_back): Update.
	* parser-defs.h (paren_depth): Don't declare.
	* parse.c (paren_depth): Remove global.
	(parse_exp_in_context): Update.
	* p-exp.y (paren_depth): New global.
	(pascal_parse): Initialize it.
	* m2-exp.y (paren_depth): New global.
	(m2_parse): Initialize it.
	* go-exp.y (paren_depth): New global.
	(go_parse): Initialize it.
	* f-exp.y (paren_depth): New global.
	(f_parse): Initialize it.
	* d-exp.y (paren_depth): New global.
	(d_parse): Initialize it.
	* c-exp.y (paren_depth): New global.
	(c_parse): Initialize it.
	* ada-lex.l (paren_depth): New global.
	(lexer_init): Initialize it.